Throughout this application various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application, in order to more fully describe the state of the art to which this invention pertains.
Conventional drug delivery technology, which in the past has concentrated on improvements in mechanical devices, such as implants or pumps, to achieve more sustained release of drugs, is now advancing on a microscopic and even molecular level. Recombinant technology has produced a variety of new potential therapeutics in the form of peptides and proteins, and these successes, have spurred the search for newer and more appropriate delivery and targeting methods and vehicles.
Microencapsulation of drugs within biodegradable polymers and liposomes has achieved success in improving the pharmacodynamics of a variety of drugs, such as antibiotics and chemotherapeutic agents [J. A. Zasadzinski, Current Opinion in Solid State and Materials Science 2, 345 (1997); D. D. Lasic, D. Papahadjopoulos, Current Opinion in Solid State and Materials Science 1, 392 (1996); D. D. Lasic, Liposomes: From Physics to Applications (Elsevier, Amsterdam (1993); T. M. Allen, Current Opinion in Colloid and Interface Science 1, 645 (1996)]. These drug delivery structures are designed to encapsulate a drug efficiently inside a polymer or lipid shell, and are administered to the patient. The drug delivery vehicles are sometimes either actively or passively targeted so that they release their entrained drug at a specific target site in the body. This targeted release of a drug has been shown to increase the effectiveness of the encapsulated drug and decrease the adverse side effects typically seen when administering the free drug. For example, unilamellar vesicles are currently used as drug delivery vehicles for a number of compounds where slow, sustained release or targeted release to specific sites in the body is desired. The drug to be released is contained within the aqueous interior of the vesicle and release is achieved by slow permeation through the vesicle bilayer. A variety of modifications of the unilamellar vesicle membrane have been attempted, including polymerizing, or crosslinking the molecules in the bilayer, to enhance stability and reduce permeation rates, and incorporating polymers into the bilayer, to reduce clearance by macrophages in the bloodstream.
There are several examples of drug delivery systems that have been developed. One example of such a vesicle structure is known as Depofoam(trademark) (i.e. Multivesicular Liposome (MVL)) [J. A. Zasadzinski, Current Opinion in Solid State and Materials Science 2, 345 (1997); M. S. Spector, J. A. Zasadzinski, M. B. Sankaram, Langmuir 12, 4704 (1996); T. Kim, S. Murdande, A. Gruber, S. Kim, Anesthesiology 85, 331 (1996)]. Depofoam(trademark) is a multivesicular particle that is created by multiple emulsification steps. A defined lipid composition is dissolved in a volatile solvent. The dispersed lipids in the solvent are vigorously mixed with water to form a first emulsion, designated a solvent continuous emulsion. This first emulsion is then added to a second water/solvent emulsion and emulsified to form a water in solvent in water double emulsion. The solvent is removed from the mixture, resulting in discrete, foam-like spherical structures consisting of bilayer-separated water compartments. The minimum size of these structures is about 5 to 10 microns. Depofoam(trademark) particles do not include a distinct bilayer structure that encapsulates the multivesicular particles, i.e. there are no individual, distinct interior vesicles. Therefore, the interior compartment must share the bilayer walls. Because of the emulsification in solvents, these Depofoam(trademark) particles are not capable of encapsulating existing vesicles, or sensitive biological materials, that degrade or denature in the presence of solvent. Production of these particles also requires high shear rates to promote emulsification. Such shear rates would degrade many biological macromolecules.
Liposomes are sealed, usually spherical, either unilamellar or multilamellar vesicles that are capable of encapsulating a variety of drugs. Liposomes are the most widely studied vesicles to date and they can be formulated with a variety of lipid and compositions that can alter their stability, pharmacokinetics and biodistribution [T. M. Allen et al., Adv. Drug Deliv. Rev., 16, 267-284 (1995)]. The lipid bilayer acts to encapsulate a drug and control its release rate. Liposomes typically include polymers inserted into the vesicle membrane in order to shield the liposomes from macrophages attempting to clear foreign objects from the body. These polymers greatly enhance the circulation time of liposomes. Liposomes can also incorporate specific binding agents on their surface in order to try to target the vesicles to a specific target organ or cell type.
A disadvantage of both multilamellar and unilamellar liposomes as delivery systems is their size, which prevents them from crossing most normal membrane barriers and limits their administration by the intravenous route. In addition, the tissue selectivity of liposomes is typically limited to the reticuloendothelial cells, which recognize them as foreign microparticulates and then concentrate the liposomes in tissues, such as the liver and spleen. A further disadvantage of the liposome system is its reliance on a single lipid membrane for controlling drug encapsulation, drug permeability, and liposome biocompatibility. It has proven quite difficult to find lipid membranes able to carry out all these tasks effectively.
Polymers have also been used as drug delivery systems. Polymer structures similar to lipid vesicles are prepared carrying an entrained drug, such as Prolease and Medisorb (Alkermes, Inc). They generally release drugs by (1) polymeric degradation or chemical cleavage of the drug from the polymer; (2) swelling of the polymer to release drugs trapped within the polymeric chains; (3) osmotic pressure effects, which create pores that release a drug which is dispersed within a polymeric network; and/or (4) simple diffusion of the drug from within the polymeric matrix to the surrounding medium.
With the drawbacks of the currently available microencapsulation vehicles, there remains a need to produce better and more efficient microencapsulation vehicles to enhance drug delivery. The present invention is directed to meeting these and other challenges.
The present invention provides novel vesosome compositions having a bilayer structure for encapsulating multiple containment units, such as multilamellar or unilamellar vesicles, polymer spheres, DNA complexes, micelles, emulsion droplets or other submicroscopic particles. These multiple containment units can contain drugs, imaging agents, DNA, emulsions, colloidal particles, enzymes, cosmetics, proteins and other diagnostic and therapeutic agents.
Further, the present invention provides a variety of new methods for encapsulating containment units, such as lipid vesicles, within an outer encapsulating bilayer membrane, and methods of controlling the number of exterior bilayers, the organization of the interior vesicles or biological materials inside the vesicles, and the structure of the encapsulating bilayer membrane. The invention further provides encapsulation methods for encapsulating both multiple individual containment units and aggregated containment units.
The encapsulating bilayer membranes of the invention can be either unilamellar or multilamellar, and are made of a variety of lipid compositions. Complex multiple chamber encapsulating structures can also be created. The size of the encapsulating bilayer membrane can be controlled either by manipulating the lipid composition of the membrane or by mechanical processing.
The encapsulated containment units (e.g. vesicles) can either be of uniform size and composition or of varied size and composition. They can be unilamellar or multilamellar. The vesicles can vary in size (as long as they are smaller than the encapsulating outer bilayer membrane structure) and can either be free-floating or aggregated to one another by ligand-receptor, antibody-antigen, or electrostatic or covalent chemical interactions. Other free-floating or aggregated colloidal particles or biological macromolecules can be encapsulated in a similar fashion.
Furthermore, the encapsulating bilayer membrane can either attach to the vesicles, vesicle aggregates, colloidal particles, colloidal aggregates, or biological macromolecules by ligand-receptor, antibody-antigen, or electrostatic or covalent chemical interactions, or the encapsulating bilayer membrane can be used to encapsulate vesicles passively i.e. without the aid of any attractive interaction. The encapsulating bilayer membrane can further be loaded with polymer lipids, or site-specific antigens (or other recognition molecules), to increase the effectiveness of drug delivery.
The exterior encapsulating bilayer membranes can regulate the permeation of the interior contents, of the containment units, at a variety of rates due to the multiple membrane permeation barriers that can be established by employing different lipid compositions. The membrane barriers of the containment units and the encapsulating bilayer membrane can also protect the interior contents from destabilizing factors such as degradation, shear, etc.
By optimizing both the exterior bilayer membrane structure and the interior containment unit compositions, the size and size distribution of the interior containment units, the overall size of the vesosome, the nature of the attachments of the interior containment units, and the type of additives to the outer bilayer membrane (such as polymers or specific recognition sites), an extremely versatile drug delivery system can be developed for a variety of applications.